Photoelectrosolographic imaging process

ABSTRACT

A PHOTOELECTROSOLOGRAPHIC IMAGING SYSTEM UTILIZING A PLATE OVERCOATED WITH A SOLUBLE LAYER. AN IMAGE IS FORMED ON THE PLATE BY FORMING AN ELECTROSTATIC LATENT IMAGE ON THE SURFACE AND TREATING THE SURFACE WITH A SOLVENT FOR THE SOLUBLE LAYER WHEREBY THE LAYER DISSOLVES AWAY SELECTIVELY IN THE UNCHARGED AREAS.

BY GAIL. D. JvlRBLls A TTORNE Y United States Patent O 3,677,750 PHOTOELECTROSOLOGRAPHIC IMAGING PROCESS Joseph Mammino, Pentield, N.Y., and Gail D. Jvirblis,

Mountain View, Calif., assiguors to Xerox Corporation,

Rochester, N.Y.

Continuation-impart of application Ser. No. 553,836, May 31, 1966, now abandoned. This application Sept. 12, 1969, Ser. No. 857,429

Int. Cl. G03g 13/00 U.S. Cl. 96-1 8 Claims ABSTRACT OF THE DISCLOSURE A photoelectrosolographic imaging system utilizing a plate overcoated with a soluble layer. An image is formed on the plate by forming an electrostatic latent image on the surface and treating the surface with a solvent for the soluble layer whereby the layer dissolves away selec` tively in the uncharged areas.

BACKGROUND OF THE INVENTION This application is a continuation-in-part of my copending application Ser. No. 553,836, tiled May 31, 1966, now abandoned.

This invention relates in general to imaging systems, and more specifically, concerns an improved photoelectrosolographic imaging system.

There has been recently developed a photoelectrosolographic imaging system capable of producing images of high quality and excellent resolution. This system is described in detail and claimed in copending applications Ser. No. 460,377, tiled June 1, 1965, now U.S. Pat. 3,- 520,681, and Ser. No. 483,675, filed Aug. 30, 1965. In a typical embodiment of this imaging system, a layer of softenable material is coated onto a conductive substrate and a softenable layer is overcoated with a fracturable photoconductive layer forming an imagable plate. The fracturable layer may be particulate and the softenable layer may be soluble in a solvent which does not attack the fracturable layer. An electrostatic latent image is formed on the surface of the fracturable layer, e.g., by uniform electrostatic charging and exposure to a pattern of activating electromagnetic radiation. The softenable layer is then softened, e.g., by dipping the plate in a soli vent. Portions of the fracturable layer which have not been exposed migrate through the softenable layer as it is softened or dissolved leaving an image on the conductive substrate conforming to a negative of the original. Those portions of the fracturable layer which do not migrate to the conductive substrate and the softenable layer are washed away with the solvent for the softenable layer. The image resulting is of high quality and of especially high resolution. Alternative embodiments are further described in the above-cited copending application.

Another recently developed imaging system, called electrosolographic imaging utilized non-photoconductive particles coated over a non-photoconductive soluble layer on a conductive substrate. Here, an electrostatic latent image is formed, for example, by corona charging through a stencil. When the imaged sheet is contacted with a solvent for the softenable layer, particles migrate to the substrate in image configuration. Unwanted particles are washed away with the soluble layer. While this imaging process does not require photoconductive materials, it is severely limited in that the charge pattern must be applied in image configuration, e.g., by corona charging through a stencil.

Each of these two imaging systems is capable of producing excellent images. However, each has limitations. The

p 3,677,750 Patented July 18, 1972 ice photoelectrosolographic imaging system described above, requires that the fracturable layer comprise a photoconductive material. Ordinarily, this layer is in the form of particles. Many of the more sensitive photoconductors do not produce desirable images. Generally, the color of the photoconductive particles is other than black so that the final image produced is not black-on-white. Also, a large proportion of the photoconductor is washed away with the soluble layer, which may make the process unduly expensive Where the photoconductive material is high priced. Also, where the particulate material has toxic properties, handling it in solution or dispersed in a solvent may be hazardous to operating personnel. 'I'he electrosolographic system described above is limited in that the electrostatic image must be originally formed in image configuration. This limits the image to those that can be produced from a stencil or some other similar means.

Various imaging systems which utilize differences in solubility in treated and untreated areas on a plate to form an image are known. Typical of these are the photopolymerization systems in which a resin surface is polymerized in light-struck areas. The polymerized or crosslinked areas are less soluble in solvents so that when the plate is treated with a solvent, the non-light-struck areas are washed away. While this system has many uses, it is not suitable for many copying purposes since a negative rather than positive copy of the original is produced. In a system described in Australian Patent 240,111, the photosensitive layer on a substrate is placed in a solvent and simultaneously is subjected to a pattern of electromagnetic radiation. It has been found that the photosensitive surface is dissolved away in proportion to the intensity of the electromagnetic radiation striking the surface. While this system is capable of producing a positive image, exposure and processing of the plate is undesirably complex since the plate must be immersed in the solvent during the exposure operation.

Thus, there is a continuing need for improved methods and materials for photoelectrosolographic and electrosolographic imaging.

SUMMARY OF THE INVENTION It is, therefore, an object of this invention to provide imaging systems overcoming the above-noted disadvantages.

It is another object of this invention to provide a photoelectrosolographic imaging system capable of producing positive images.

It is still another object of this invention to provide a photoelectrosolographic imaging system utilizing a wide range of photoconductive materials.

It is another object of this invention to provide an imaging system producing a positive image which may be easily reversed to form a negative image.

It is still another object of this invention to provide a. simple method of preparing relief printing plates.

It is yet another object of this invention to provide an imaging method producing easily intensified relief images.

The above objects and others are accomplished fundamentally by providing a photoelectrosolographic imaging system utilizing a plate comprising a substrate overcoated with a soluble layer. An image is formed on such a plate by forming an electrostatic latent image on the surface of said plate and treating the surface of said plate with a solvent for the soluble layer whereby the layer dissolved away selectively in exposed areas. In one embodiment, the soluble layer is photoconductive and the electrostatic latent image is formed by uniformly electrostatically charging the surface of the plate and exposing said surface to a pattern of activating electromagnetic radiation. In a second embodiment, the soluble layer is insulating and the electrostatic latent image is formed directly, e.g.,

by corona charging through a stencil or by transfer of an electrostatic latent image to the surface in the manner described by Walkup in U.S. Pat. 2,833,648. The electrostatic latent image may be formed on the surface of the soluble layer an appreciable time before the image is to be developed. This time is limited only by the dark dis charge rate of the layer. Where the layer is an effective insulator, this period may be several days. The image produced by this system is a positive, conforming to the original or to the directly formed electrostatic latent image. It has been found that this positive image may be reversed to form a negative light scattering image by merely heating the imaged plate to the softening point of the soluble layer. At this temperature, the background areas frost in the manner described in U.S Pat. 3,196,- l1. Where originally the image was in the form of raised positive areas against lower background areas, now the image appears as frosted background areas and transparent positive image areas.

BRIEF DESCRIPTION OF THE DRAWINGS The electrosolographic plate and imaging process of this invention may be further understood upon reference to the drawings, wherein:

,FIG 1 shows a cross-section through a photoelectrosolographic plate before imaging,

FIG. 2 schematically shows the electrostatic charging of the plate of FIG. 1,

PIG. 3 shows the exposure of the plate of FIG. l to a pattern of activating electromagnetic radiation, l

FIG. 4 shows development of a positive electrosolographic image,

FIG. 5 shows a cross-section of the plate bearing the positive image,

FIG. 6 shows the frosting of background areas to produce a negative light scattering image, and

FIG. 7 shows a cross-section through the plate bearing the negative light scattering image.

Referring now to FIG. 1, there is seen a cross-section through a plate suitable for use in the process of this invention. This plate consists of a Substrate 1 coated with a layer of softenable material 2. Substrate 1 may cornprise any suitable conductive or non-conductive material. Typical conductive materials include metallic sheets such as steel, brass, aluminum; transparent sheets having conductive coatings thereon such as tin oxide coated glass and paper or resin sheets or `films having conductivity increasing additives where necessary. Typical insulating materials include polyethylene, polypropylene, polyethyleneterephthalate, cellulose acetate, paper, plastic coated paper, such as polyethylene coated paper, vinyl chloridevinylidene chloride copolymers and mixtures thereof. Mylar (a polyester formed by the condensation reaction between ethylene glycol and terephthalic acid available from E. I. du -Pont de Nemours & Co., Inc.) is preferred because of its durability and excellent insulative properties. For purposes of illustration only, Substrate 1 is conductive. Softenable layer 2 will comprise either -a soluble organic photoconductive material where it is desired to form the image by projection or a soluble insulating material where the image is formed directly, e.g., by corona charging through a stencil. Typical photosensitive layers include organic photoconductors in a resin binder, soluble photoconductive polymers, charge transfer complexes of certain aromatic resins and Lewis acids, and mixtures thereof. Typical organic photoconductors include anthracene, 2,5-bis-(p-aminophenyl)-1,3,4oxadiazole; 2aryl4 arylidene-oxazolones; 2,5 bis (p-amino-phenyl)-1,3,4tri azoles; quinazilines; thiazolidones; triphenylamines; and mixtures thereof. Typical aromatic resins which may be sensitized with Lewis acids include: polyvinyl carbazole, epoxy resins, phenoxy resins, phenol-formaldehyde resins, polystyrenes, polycarbonates, polysulfones, polyphenylene oxides and mixtures thereof. Typical Lewis acids which may be used to sensitize the above resin include 2,4,7-trinitro-9Jlluorenone; 4,4(dimethylamino) benzophenone; chloranil; 1,3,5-trinitrobenzene and mixtures thereof. Any suitable soluble insulating resin may be used in the softenable layer. Typical insulating resins include polyolens; vinyl and vinylidene resins such as polystyrene, polyrnethylmethacrylate; heterochain thermoplastics such as polyamides, polyurethanes, polycarbonates; phenolic resins; amino resins; silicone resins; and mixtures thereof.

The step of uniformly electrostatically charging the surface of the plate in the photoconductive embodiment thereof is shown schematically in FIG. 2. In this instance, the plate is charged by means of corona charging head 3 which deposits a uniform positive charge on the surface of the plate as it passes across the plate. Typical corona charging methods are described by Walkup in UJS. Pat. 2,777,957 and by Carlson in U.S. Pat. 2,588,699.

FIG. 3 shows schematically the exposure of the charged plate to a pattern of activating electromagnetic radiation. The plate in this instance is exposed by means of a lens 5, black-and-white transparency 6 and light source 7. The photoconductive softenable layer 2 appears to discharge in light struck areas leaving surface charge in areas 7 where light does not strike the plate. Thus, a positive electrostatic latent image is formed on the surface of the plate. In the embodiment of this invention wherein softenable layer 2 is not photosensitive, the electrostatic image must be formed directly; for example, by corona charging through a stencil.

FIG. 4 shows schematically the step of developing the electrostatic latent image previously formed on the surface of layer 2. The plate is immersed in container 8 which contains a solvent for layer 2. Any suitable solvent may be used. It is preferred that the solvent be substantially insulating so as not to degrade the electrostatic image. Typical solvents include carbon tetrachloride, cyclohexane, hexane, heptane and mixtures thereof. As the plate remains in the solvent, one may observe the differential dissolving of background areas. While the solvent does attack the image areas, it attacks the background areas much more rapidly. When a Well-dened image is observed, the plate is withdrawn from the solvent bath and dried. It the plate is left in the solvent for an extended period, the solvent will dissolve layer 2 away entirely.

FIG. 5 shows a cross-section through the imaged plate. A positive image is produced consisting of raised areas 7 which conform to the surface electrostatic latent image formed on the plate previously. Ordinarily, the resin is not entirely dissolved away in background areas, a thin film remaining over the plate. This plate may be viewed by reflected light. A colorant may be applied to the raised areas, if desired, in order to enhance contrast between image and background areas. For example, the plate may be lightly pressed against a conventional rubber-stamped inking pad. This plate is also suitable for printing in the conventional relief printing mode. If it is preferred that the linal image be negative rather than positive an apparent reversal of the image may be obtained.

FIG. 6 shows schematically a method for obtaining a light scattering or diffusing surface in background areas of the plate of FIG. 5. The plate is heated to the softening point of the softenable layer, in this instance, by means of heated platen 9. 'This plate is maintained at a temperature slightly above the softening temperature of the softenable layer 2. As the softenable layer reaches its softening temperature, background areas are seen to deformy in a frostlike manner. The mechanism of frost surface deformation is described in detail in U.S. Pats. 3,196,011 and 3,196,008. The plate appears now as shown in section in FIG. 7. Clear areas 7 are seen against a white background 10. By projection, these background areas appear black and areas 7 white or transparent. This gives the appearance of a negative image. It is not fully understood why background areas rather than image areas 7 frost. This is especially surprising since after the exposure step shown schematically in FIG. 3 a surface electrostatic charge remains only in areas 7. As discussed in the above cited copending applications, frost ordinarily appears only in charged areas. However, it is thought that during the exposure steps shown in PIG. 3 charge does not migrate entirely to conductive substrate 1 but rather is merely driven below the surface of plate 2. Then, during the developing step shown in FIG. 4 charge in areas 7, being a surface charge, is easily dissipated. The more tightly bound charge in background areasremain and provides the field necessary for frost in the step shown schematically in FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following examples will further point out the imagable plate and imaging process of this invention. Parts and percentages are by weight unless otherwise indicated. The examples below should be considered to illustrate various preferred embodiments of the invention.

EXAMPLE I About 9 parts of Bakelite resin 5254, a p-phenylphenol phenolic resin, available from Union Carbide Corporation is dissolved in a solvent blend made up of about l parts toluene and about 8 parts acetone. About l part of 2,4,7-trinitro-9-uorenone is added to the resin solution and the mixture stirred until all of the materials are well dispersed. This solution is then coated onto a 5 mil sheet of ll45-H 19 aluminum foil, available from the Aluminum Company of America, by means of a Bird applicator, available from Bird & Sons, Inc., to a dry thickness of about 5 microns. The coated plate is forced air-dried at about 100 C. for about 5 minutes. The plate is charged to a negative potential by means of corona discharge as described by Carlson in U.S. Pat. 2,588,699. The corona unit is maintained at a potential of about 7500 volts. The charged plate is exposed by projection with a positive black-and-White transparency by means of a Solar enlarger, available from Burke & James Co. A Z50-watt General Electric Photoood BBA Lamp, 3400" K. color temperature, is used. Total exposure is about 200 footcandle-seconds. The plate bearing the resulting latent electrostatic image is immersed in a developer solvent consisting of carbon tetrachloride. The plate is observed through the surface of the developer liquid and the plate is removed when it is observed that the background areas have substantially completely dissolved away but image areas have not appreciably dissolved This developing step requires about 5 seconds in the solvent solution. An excellent image consisting of raised positive areas against a depressed background is observed on the plate.

EXAMPLE II The image plate produced in Example I is placed on a heated platen maintained at a temperature of about 300 C. As the temperature of the plate reaches the softening temperature of the resin, uniform surface deformation consisting of fine surface folds or wrinkles having the appearance of frost appears uniformly across background areas. The plate now bears an. image appearing as a negative of the original transparency. The image consists of clear positive image areas and frosted, light-scattering background areas.

EXAMPLE III A resin solution is prepared by dissolving about 9 parts of Dow ET-693, a phenolic resin available from the Dow Chemical Company, in about parts toluene. About 1 part 2,4,7-trinitro-9-uorenone is added to the resin solution and the mixture is stirred until all of the materials are well dispersed. This solution is coated onto the conductive surface of a sheet of NESA glass, tin oxide coated glass available from the Pittsburgh Plate Glass Company. The coating has a dry thickness'of about 8 microns. This plate is charged, exposed and developed as in Example I. A positive image results, consisting of raised areas against a depressed background, the raised areas conforming to a positive copy of the original transparency.

EXAMPLE IV The imaged plate prepared in Example III is placed on a heated platen maintained at a temperature of about 300. As the temperature of the plate reaches the softening point of the resin, uniform frost appears across background areas of the plate. The image now appears as positive transparent areas against a frosted, light-scattering background. The plate is placed in a conventional slide projector and an image is projected on a white surface. The projected image conforms to a negative of the original transparency in that the transparent image areas appear white and the frosted areas black.

EXAMPLE V Another imaged plate is prepared as in Example III above. The imaged plate having raised imaged areas on a depressed background is brought into gentle contact with a sheet of aluminum having a thin coating of India ink on the surface thereof. When the plate is removed, it is seen that the raised positive image areas have become coated with a uniform layer of India ink. This ink may be allowed to dry resulting in an intensified positive image. Alternatively, the plate bearing the Wet ink may be pressed against a receiving sheet, such as paper, to transfer an image in the manner of relief printing plate.

EXAMPLE VI A resin solution is prepared by initially dissolving about l0 parts Piccotex 100, a blend of polymerized styrenes and vinyl toluenes, available from the Pennsylvania 1ndustrial Chemical Company, in about 20 parts toluene. About 1 part 2,4,7-trinitro-9 uorenone is added to the resin solution and the mixture is stirred until al1 of the materials are well dispersed. This solution is ow coated onto an aluminum sheet to a dry thickness of about 5 microns. The plate is charged, exposed and developed as in Example I above. A positive image consisting of raised positive areas against a depressed background conforming to the original transparency results.

EXAMPLE VII The imaged plate prepared in Example VI is placed on a heated platen maintained at a temperature of about 300. As the plate temperature reaches the softening temperature of the resin, uniform frost develops in background areas. The resulting image appears as positive transparent areas against a frosted, light-scattering background. By projection, this image appears as a negative of the original transparency.

EXAMPLE VIII A resin solution is prepared by dissolving about 9 parts Bakelite resin 5254 in about 20y parts toluene. About 1 part 1,3,6,8-tetranitrocarbazole is added to the resin solution and the mixture is stirred. The solution is then flow coated onto an aluminum sheet and dried. The resin has a thickness of about 5 microns. The plate is charged, exposed and developed as in Example I above. A positive image consisting of raised areas against a depressed background results.

EXAMPLE IX A plate is prepared by dissolving about 9 parts Bakelite resin 5254 and about l part 2,5-bis(paminophenyl) 1,3,4-oxadiazole, available from Kalle Inc., in about 20 parts toluene. This solution is flow coated onto an aluminum substrate and dried. The coating has a dry thickness of about 8 microns. The plate is charged, exposed and developed as in Example I. A positive image consisting of raised image areas against a depressed background is obtained.

EXAMPLE X A plate is prepared as in Example IX above except that about `0.1 part of Rhodamine B Base, a Xanthene dye available from Allied Chemical is added to the resin solution. The plate thus produced is charged, exposed and developed as in Example I above. The positive image consisting of raised image areas against a depressed background conforming to the original results. This dye-sensitized plate appears to develop slightly more rapidly in the solvent developer solution than did the unsensitized plate of Example X.

EXAMPLE XI A resin solution is prepared by dissolving about 9 parts of Bakelite resin 5254 in about 20 parts toluene. This solution is flow-coated onto an aluminum substrate and dried. The coating has a dry thickness of about microns. An electrostatic latent image is formed on the surface of the plate by corona charging through a stencil consisting of a metal sheet with an image forming opening therein. The plate bearing the thus formed electrostatic latent image is immersed in a ybath of cyclohexane for about 5 seconds. The surface of the plate is observed through the solvent. The resin coating is seen to dissolve away primarily in background are-as. When background areas have been substantially entirely dissolved, but before any appreciable dissolving of image areas, the plate is removed from the solvent and dried. The image on the plate consists of positive raised areas against a depressed background and conforms to the opening in the stencil.

EXAMPLE XII The resin solution of Example X-I is flow-coated onto a 2 mil thick polyethylene terephthalate sheet available from E. L du Pont de Nemours & Co., Inc. under the trademark Mylar. The coating has a dry thickness of about 5 microns. The coated substrate is then electrically charged by passing it between upper and lower corona discharge devices in accordance with the apparatus and process described in U.S. Pat. 2,885,556. The charging method places a positive potential of 800 volts on the coated surface and a positive potential of 300 volts on the opposite surface. Inserted between the surfaces of the su-bstrate and each corona discharge device is a stencil consisting of a metal sheet with an image forming opening therein. The plate bearing the thus formed image is immersed in a bath of cyclohexane for about 5 seconds. The surface of the plate is observed through the solvent. The resin coating is seen to dissolve away primarily in background areas. When background areas have been substantially entirely dissolved, but before any appreciable dissolving of image areas, the plate is removed from the solvent and dried. The image on the plate consists of positive raised areas against a depressed background and conforms to the opening in the stencil.

Although specific components and proportions have been described iu the above examples, relating to methods of preparing electrosolograp-hic and photoelectrosolographic plates and the methods of imaging using said plates, other suitable materials as listed above may be used with similar results. In addition, other materials may be added to the softenable layer or to the developer solution to synergize, enhance or otherwise modify their properties. For example, the photoconductive softenable layer may include evarious spectral or electrical sensitizers.

Other modifications and ramiiications of the present invention will occur to those skilled in the art upon a reading of the present disclosure. These are intended to be included within the scope of this invention.

What is claimed is:

1. An imaging process which comprises the steps of:

(a) providing a plate comprising a substrate overcoated with a layer comprising a soluble organic material selected from the group consisting of electrically insulating resinous materials and photoconductive insulating materials |(b) forming an electrical charge pattern in imagewise conguration on said plate and subsequently (c) developing said latent image in the absence of imagewise illumination by immersing said plate in a substantially insulating solvent for said soluble layer for a period suilicient to dissolve away background areas whereby an image of raised areas of said organic material against a depressed background is formed.

2. The process of claim 1 wherein the substrate is electrically conductive.

3. The process of claim 1 wherein said substrate is electrically insulating.

4. The imaging process of claim 1 wherein said layer comprises a soluble organic photoconductive material, said substrate is electrically conductive and said electrical charge pattern is formed by substantially uniformly electrostatically charging the surface of said plate and exposing said surface to a pattern of actinic electromagnetic radiation.

5. The imaging process of claim 1 wherein said electrical charge pattern is formed on said plate by applying an electrostatic charge thereto in image configuration.

6. The imaging process of claim 1 with the additional step of heating said image plate to the softening temperature of said soluble material whereby frost surface deformation occurs in the background areas.

7. The imaging process of claim 1 wherein a colorant is applied to the raised areas on said imaged plate whereby an intensified positive ima-ge is formed.

8. The imaging process of claim 7 wherein said colorant is transferred from said raised areas to a receiving sheet by pressure contact.

References Cited UNITED STATES PATENTS 3,515,549 6/ 1970 Bixby 96-1.5 3,520,681 7/1970 Goife 96-1 3,305,359 2/ 1967 Delmont 96-1 3,337,339 8/1967 Snelling 96-1 3,161,505 12/1964 Tomanek 96-1.5 X 3,196,008 7/ 1965 Mihajlow et al 96-l.1 X 3,408,181 10/ 1968 Mammino 96-1 X 3,512,966 5/ 1970 Shattuck et al 96--1.6 X

FOREIGN PATENTS 240,1 1 l 8/ 1962 Australia 96-1 OTHER REFERENCES Cassiers, Memory Effects in Electrophotography, Journal of Photographic Science, vol. 10, 1962, pp. 5 7-64.

CHARLES E. VAN HORN, Primary Examiner l. R. MILIJER, Assistant Examiner 

